In many sewage works there is a large variation in the flow throughout the day, often a 12:1 ratio, and it is not unusual for the pollution load to vary proportionately with the flow. If the sewage works is to be operated with a constant dissolved oxygen concentration in the activated sludge tanks, it follows that the rate of oxygen addition must be capable of being varied in the same ratio throughout the 24 hour period. It is therefore advantageous for any device used for dissolving oxygen in the sewage to be capable of this kind of flexibility. It has been proposed to dissolve gas in an open body of liquid by injecting gas in a flow of the liquid passed through a downwardly divergent conduit which opens into the body of the liquid with the bubbles of injected gas dissolving to some extent in the liquid as it passes through the conduit.
These proposals however have required that the buoyant velocity of the bubbles of gas in the conduit is always less than that of the flow of liquid through the conduit so that there is no pocket of undissolved gas formed at the upper end of the conduit. However, the larger size of gas bubbles which are bound to be created, for example by coalescence of a number of bubbles formed in the downwardly flowing stream of liquid in the conduit, will rise upwardly in the conduit to its upper end. The applicants have found that in such a method where the incoming liquid falls through a clearly defined and relatively still gas pocket at the top of the conduit, the amount of oxygen in the solution is relatively small. The Applicants have now discovered that by maintaining a turbulent gas/liquid contact zone in the conduit considerably higher oxygen transfer rates can be achieved. Indeed the Applicants have discovered that if substantially the whole contents of the conduit are frothing very high oxygen transfer rates can be achieved.
A turbulent gas/liquid contact zone can be created by causing the incoming flow of sewage to impinge on the liquid surface in the conduit at a rate to cause the required turbulence to generate bubbles of gas entrained from the pocket of undissolved gas formed above the liquid space in the conduit. It is also thought that the larger size bubbles, of undissolved gas, which is introduced into the stream before it enters the conduit, rise in the liquid in the conduit creating a turbulence of upward and downward currents in the liquid in the conduit thereby greatly extending the gas/liquid contact time.